Chloroprene rubbers, which are superior in mechanical properties, weather resistance, and flame resistance, have been used widely as a material for industrial rubber products. In recent increasing demand for improvement of the properties of industrial rubber products, higher heat resistance is also demanded in addition to the favorable mechanical properties, weather resistance, and flame resistance described above. In particular, automobile rubber parts are exposed during use to significantly higher-temperature environments because of improvement in performance of engine and reduction in size of the body and thus, high heat resistance is required for the rubber products used.
On the other hand, as chloroprene rubbers contain double bonds therein even after vulcanization, they have a disadvantage of low heat resistance. Accordingly, various studies have been in progress to overcome the disadvantage of low heat resistance (see, for example, Patent Documents 1 to 5). For example in the case of the chloroprene rubber composition described in Patent Document 1, zinc powder or zinc powder and water was added to the chloroprene rubber for improvement of the heat resistance.
Alternatively in the case of the rubber compositions described in Patent Documents 2 to 4, carbon black having a LC (average stacking height in the C-axis direction to the crystal-lattice plane) of 2 nm or more and zinc powder or a mixture thereof with a particular plasticizer is added to the chloroprene rubber for improvement of the heat resistance of the vulcanizate. Alternatively in the case of the chloroprene rubber composition described in Patent Document 5, a carbon black having a number-average particle diameter and a DBP oil absorption in particular ranges is added to a xanthogen-modified chloroprene rubber and/or a mercaptan-modified chloroprene rubber for improvement of heat resistance without deterioration in mechanical properties and others.